A III-V compound semiconductor material containing nitrogen (hereinafter referred to as a “nitride semiconductor material”) has a band gap corresponding to the energy of light having wavelengths from the infrared region to the ultraviolet region. Hence, the nitride semiconductor material is useful for the material of a light emitting element that emits light having wavelengths from the infrared region to the ultraviolet region, the material of a light emitting element that receives light having wavelengths in the regions and the like.
The bond between the atoms of the nitride semiconductor material is strong, the breakdown voltage of the nitride semiconductor material is high and the saturation electron velocity of the nitride semiconductor material is high. Hence, the nitride semiconductor material is also useful as the material of an electronic device such as a high-frequency transistor that is resistant to high temperature and that produces a high output. Furthermore, since the nitride semiconductor material hardly harms the environment, attention is focused on the nitride semiconductor material as an easy-to-handle material.
In a nitride semiconductor light emitting element using such a nitride semiconductor material, as a light emitting layer, a quantum well structure is generally adopted. When voltage is applied to the nitride semiconductor light emitting element, an electron and a hole are recombined in a well layer forming the light emitting layer, and thus light is generated. The light emitting layer may be formed with a single quantum well (SQE) structure or may be formed with a multiple quantum well (MQW) structure in which the well layer and a barrier layer are alternately deposited.
It is known that advantageously, in the nitride semiconductor light emitting element, as compared with an AlGaInP LED (light emitting diode), the temperature is less varied by variations in ambient temperature. A conventional nitride semiconductor light emitting element disclosed in patent document 1 is a light emitting diode, on an n-type nitride semiconductor layer formed on a sapphire substrate, an active layer having InxGa1-xN (0.4<x<1) sandwiched between an undoped GaN layer and a p-type AlGaN layer is provided and the ratio between In and Ga in the active layer is set such that yellow or amber light is emitted. Advantageously, in the nitride semiconductor light emitting element configured as described above, as compared with a conventionally and widely used amber light emitting element using AlInGaP, a large light emission output is produced, and the light emission output is less dependent on temperature as compared with the AlInGaP amber light emitting element. In the nitride semiconductor light emitting element described above, when it is driven with a current of 20 mA, at a temperature of 80° C., 90% of the light emission output at room temperature is maintained as its light emission output.
In patent document 2, it is described that it is possible to obtain the temperature characteristics of such a light emission output because of the unique characteristics of its material. Specifically, in the conventional amber light emitting element using AlInGaP, in order to perform lattice matching on a GaAs substrate and an AlInGaP epitaxial layer used as the substrate, the amount of offset of a band gap between the active layer and a clad layer is inevitably lowered. Consequently, as the temperature is increased, the amount of overflow of carriers is increased, and thus the temperature dependence is increased.
Patent document 2 also discloses a conventional nitride semiconductor light emitting element in which an AlON buffer layer is formed on the surface of the substrate by a sputtering method, and on the surface of the AlON buffer layer, a GaN layer is grown by a depression MOCVD method. It is disclosed that in the nitride semiconductor light emitting element described above, an X ray (rocking curve) half-value width of an AlN (002) plane of an AlNO buffer layer 2 in a sample where a satisfactory GaN characteristic was obtained was 300 arc sec or less and the refractive index was 2.08 or less. It is thought that as the value of the X ray half-value width is lowered, a satisfactory crystal where the number of dislocations is lowered is obtained.
In a nitride semiconductor light emitting element disclosed in patent document 3, an active layer contains an n-type impurity, and the n-type dopant concentration of the active layer located on the side of an n-layer is higher than the n-type dopant concentration of the active layer located on the side of a p-layer. Thus, it is possible to compensate for the supply of a donor from the side of the n-layer to the active layer and thereby enhance a light emission output.